1. Field of the Invention
The invention relates in general to a digital-to-analog converting circuit, and more particular, to a digital-to-analog converting circuit of a display.
2. Related Art of the Invention
The earliest dynamic image that human beings have seen is saw was the documentary movie. Later on, When the invention of the cathode ray tubes successfully induces brought on the production of commercial television, which then becomes thethis became an almost necessary appliance for almost every household. The cathode ray tubes are then were further applied as to the desktop monitor and dominated computer industry for decades. However, cathode ray tubes have problems with the radiation problem and the massive space occupied by the internal electron gun, and the displays using the cathode ray tubes cannot be made thinner and lighter.
The above problems triggered the researchers to commence the research on flat panel displays. Such fields includes liquid crystal display (LCD), field emission display (FED), organic light emitting diode (OLED), and plasma display panel (PDP).
The organic light emitting diode, also referred to organic electro-luminescence display (OELD), is a self-illuminant device. The characteristics of organic electro-luminescence displays include low DC driving voltage, high brightness, high efficiency, high contrast, thin, and high degree of freedom for three primary colors, including red (R), green (G) and blue (B) to white. Therefore, the organic electro-luminescence display has becomes the leading technology of flat panel display for the next generation. In addition to the advantages of thin dimension, high resolution, self-illumination, fast response speed and power-saving cold light source, the organic electro-luminescence display is further characterized in wide viewing angle, promising color contrast, and low fabrication cost. Therefore, the organic light emitting diode has been broadly applied as the backlight source of liquid crystal display or indicating panel, cellular phones, digital camera, and personal data assistant.
In terms of driving method, the organic light emitting diode can be classified into the passive matrix driving type and the active matrix driving type. The passive matrix driving type of organic light emitting display is advantageous in that its simply simple structure that does not require the application of thin-film transistor (TFT). Therefore, the fabrication cost of the passive matrix driving type of organic light emitting diode is relatively low. But such type of organic light emitting diode is not suitable for use in high-resolution applications. In the development of a large panel diode, the problems of increasing power consumption, reduced device lifetime, and poor display quality may occur. In contrast, the active matrix type of organic light emitting diode meets the requirement of a large display panel while retaining the characteristics of wide viewing angle, high brightness, and fast response time. However, such a type of organic light emitting diode is relatively more expensive.
In the driving source viewpoint, the flat panel display can be divided into voltage driving type and current driving type. The voltage driving type is typically applied to thin-film transistor liquid crystal display (TFT LCD). By inputting different voltages to data lines, different gray scales are resulted to achieve full color display. The current driving type is normally applied to the organic light emitting diode display. That is, by inputting different current to data lines, different gray scales can be obtained to achieve the full color display.
For the current driving type organic light emitting diode, the data current is provided from the digital-to-analog converting circuit of the data driver to drive the pixels of the display illuminating display. The circuit diagram of the conventional digital-to-analog converting circuit is shown in FIG. 1. As shown in FIG. 1, the digital-to-analog converting circuit uses thin-film transistors TDM 102, TD1 104, TD2 106, TD3 108 and TD4 110, of which the channels lengths (L) are the same, while the channel widths (W) are different. That is, the dimensions of the thin-film transistors used in the digital-to-analog converting circuit are different from each other. Due to process inconsistency, the threshold voltages for the thin-film transistors with different dimensions are become inconsistent should process deviation occur. As a result, the output data current is deviated, to affecting the brightness of the organic light emitting diode.